Organic Light Emitting Diodes (OLEDs) are useful in electronic displays, building lighting, signage, and other applications where efficient, lightweight, thin form-factor light sources are needed. An OLED is formed by sandwiching a fluorescent or phosphorescent organic film between two electrodes, at least one of which is transparent. Holes from the anode and electrons from the cathode recombine in the organic film and produce light. If the organic film is a polymer film the device is a polymer-OLED or p-OLED. It is known in the art how to improve efficiency of OLEDs and p-OLEDs by inclusion of various other layers in the sandwich structure, including but not limited to hole injection layers, hole transport layers, buffer layers, electron injection layers, electron transport layers, hole blocking layers, electron blocking layers, exciton blocking layers, optical layers to increase light extraction efficiency, and the like. It is also known in the art that the properties of the organic film, or emissive layer, must be carefully designed to 1) allow transport of holes, 2) allow transport of electrons, 3) prevent non-radiative decay of the excited state, and 4) ensure that no irreversible chemical reactions occur during device operation. Requirements 1-3 relate to device efficiency and requirement 4 relates to device lifetime. The emissive layer will often be comprised of several substances or components, including one or more charge carriers, a fluorescent or phosphorescent material, and a more or less inert matrix.
While theory suggests that OLEDs and p-OLEDs can have high efficiencies, commercial devices still have lower efficiencies than conventional fluorescent bulbs. In practice, the efficiency of a device is dependent on color and is related to the sensitivity of the human eye, so that green devices are inherently more efficient than red or blue emitting devices, however, improvement in efficiency of all colors is desired. One cause of low efficiency is energy transfer from the excited emissive compound (whether it be fluorescent or phosphorescent, small molecule, or polymer) to a material having a lower energy excited state. Materials with lower energy excited states may be, for example, impurities, defects, or excimers. It often occurs that the matrix has a first triplet excited state that is lower in energy or only slightly above the emissive material's excited state and a first singlet-excited state that is higher than the emissive material's excited state. It would be desirable to reduce or eliminate energy transfer from the desired excited state to other lower energy excited states and to eliminate energy transfer from the desired excited state to the triplet state of the matrix material.
The decreasing brightness of OLEDs and p-OLEDs as a function of time is the major obstacle to commercial application. Many factors affect lifetime. An important factor appears to be the redox stability of the emissive layer (that is, the stability of the reduced and oxidized states of the materials in the emissive layer). While not wishing to be bound by theory, it is believed that as holes propagate through the emissive layer they take the form of cations or radical cations. A radical is a molecule having an odd number of electrons and may be charged (an anion or cation) or neutral (a free radical). Radicals are generally more reactive than molecules with an even number of electrons. As electrons propagate through the emissive layer, they take the form of anions or radical anions. Radical cations may dissociate into a cation and a free radical, while radical anions may dissociate into an anion and a free radical, Cations, radical cations, anions, radical anions, and free radicals are all reactive species and may undergo unwanted chemical reactions with one another or with nearby neutral molecules. Such chemical reactions alter the electronic properties of the emissive layer and can lead to decreases in brightness, efficiency, and (ultimately) device failure. For this reason, it would be desirable to reduce or eliminate chemical reactions of these active species in OLEDs and p-OLEDs.
Even the most promising p-OLED materials are limited by short lifetimes. For example, copolymers of methylene-bridged polyphenylenes (also called polyfluorenes) and other arylene units, Q (such as 4,4′-triphenylamine, 3,6-benzothiazole, 2,5(1,4-dialkoxyphenylene), or a second bridged biphenyl unit) are frequently used in p-OLED applications. While green emitting p-OLEDs based on such polyfluorene copolymers have been reported to have lifetimes of over 10,000 hours, red and blue emitting p-OLEDs based on these systems are shorter lived. Lifetime is generally measured as the time to half brightness at a set current density, starting at 100 cd/m2. In fact, the lifetimes of the best polyfluorene blue phosphors are not suitable for commercial p-OLED applications. For this reason, it would be desirable to improve emissive materials, especially those that emit in the blue color range.
General structures 1 and 2 for polyphenylene (top, General Structure 1) and methylene bridged polyphenylene (bottom, General Structure 2), where Q is an arylene.
In polyarylene-type green and red emissive polymers (including polyfluorene as a subclass of polyarylene) the emissive center is typically a special repeat unit selected to have a first singlet-excited state of appropriate energy to emit green or red. In polyarylene type blue emissive polymers (including polyfluorenes) the emissive center is typically one or more adjacent phenylene (or bridged biphenylene) repeat units. In this case, the phenylene (or bridged biphenylene) backbone has the lowest singlet-excited state out of all the repeat units or other materials present. That is, the majority repeat unit is the emitter. This means that excited states spend most of their time on the majority repeat unit. Since excited states are more reactive than ground states the majority repeat units are prone to undergo undesired reactions.